1. Field of the Invention
This invention relates to luminescent devices suitable for use in displays and lighting. Such find use as flat panel displays, lighting and backlights for liquid crystal flat panel displays. The flat panel display technology of this invention is useful for EL, plasma, LCD and FED displays.
2. Description of the Prior Art
In the production of electroluminescent displays, such as AC thin film electroluminescent displays, phosphors are currently applied by chemical vapor deposition, e-beam, evaporation, or sputtering techniques which results in films which are, at best, polycrystalline. In electroluminescent devices, particulate phosphors or single crystal phosphors are desirable due to the low number of defects occurring during manufacture and due to the fact that they can fully compensate for dopants. In a conventional device the phosphor is sandwiched between two thin film dielectrics which are deposited in a similar manner. A voltage is applied and the voltage is divided between the dielectrics and the phosphor. When sufficient voltage is applied electrons move in the phosphor and excite the dopant, thus giving off light. The amount of voltage on the phosphor determines how much light is obtained. The higher the dielectric constant, the more voltage which can be applied to the phosphor. Since sol gels have very high dielectric constants, by forming electoluminescent devices according to this invention, one can operate at lower voltages or use thicker layers. For particulate phosphors, which generally have a larger size than the thickness of the standard phosphor layer, similar operating voltages would be expected. This would mean brighter, more efficient displays with a much wider choice of materials for color. The use of the liquid sol gel techniques for this invention means that the cost of the devices would decrease significantly since devices could be made without the use of large expensive vapor deposition equipment. AC devices, which are currently manufactured by the aforementioned chemical vapor deposition, e-beam, evaporation, or sputtering techniques, have high luminescence and good lifetime. Thick film DC devices have low luminescence with poor aging characteristics, however, in operation the thick film devices have DC conduction through the device and the AC devices do not.
This invention allows the use of particulate phosphors in an encapsulating medium which can protect the phosphor and provide dielectric strength or conduction as required. It allows the coating of large substrates with uniform phosphor coatings by spraying, spinning or silkscreening which are standard processes for other uses. This means that patterning can also be accomplished using standard photolithographic techniques. For color displays this patterning would reduce costs by removing the process steps required to pattern the phosphor by etching. Using particulate phosphors means a larger number of phosphors is available for color EL displays since using particulate phosphors eliminates the thin film deposition step. High quality electroluminescent devices can be made for the cost of the thick film devices which are currently used for low level lighting and LCD backlights. A patterned EL or fluorescent backlight device would be used to improve the efficiency of liquid crystal displays by removing the need for color filters. This invention is also applicable for the production of plasma devices. For plasma devices there is a great advantage in the processing and reliability of the device. It is known to use particulate phosphors in plasma devices. The invention improves on these by providing a sol gel dielectric to protect the phosphors from the plasma in the device and by removing some of the contaminants used in the prior art deposition process of the phosphors. It also seals the phosphor so it cannot contaminate the vacuum or the electrodes. In an alternate embodiment, a dielectric can also be used to cover one of the electrodes. The invention is also useful for producing fluorescent lights and backlights. For plasma displays and fluorescent backlights the encapsulation composition gives improved life by protecting the phosphor from the plasma. It will also seal the phosphor thus preventing contamination of the vacuum and the electrodes. The invention also finds use in electron beam excitation applications. In cathode ray excitation applications, advantages include sealing the phosphor from the vacuum and ease of patterning high resolutions. These advantages are most useful for the field emitter displays with a small volume vacuum envelope and with a very small gap between the emitter and the screen with the luminescent display. In this application a somewhat conducting sol gel film is required so that the electrons may leak off the device. The encapsulation of the phosphor means that phosphors which normally would contaminate the emitters can now be used.